The application claims the benefit priority of European Application number 01401375.9 filed May 25, 2001.
1. Field of the Invention
The invention is directed to an optical amplifier, and in particular, to an optical amplifier that includes at least two semiconductor optical amplifiers and an optical signal isolator.
2. Technical Background
The continuous growth of bandwidth requirements in optical-based communication systems has resulted in a large demand for systems able to operate outside the amplification band provided by erbium-doped fiber amplifiers. Erbium-doped fiber amplifiers effectively operate over a limited wavelength band. Depending on amplifier configuration and fiber composition, erbium-doped fiber can be used for amplification in a wavelength band extending from 1530 nm to 1620 nm, although at least three different erbium-doped fiber amplification configurations would be required to cover this entire range.
Other rare earth-doped fiber amplifiers have been used for amplification outside the erbium wavelength and from 1530 nm to 1620 nm. These other rare earth-doped amplifiers include thulium-doped amplifiers operating from 1440 nm to 1510 xcexcm, praseodymium-doped amplifiers operating from 1250 nm to 1310 nm, and neodymium-doped amplifiers operating from 1310 nm to 1350 xcexcm. Each of these rare earth-doped amplifiers exhibit very low efficiency as well as other technical problems associated with each particular kind of dopant when compared to erbium-doped amplifiers.
Rare earth-doped amplification systems cover the available transmission window of traditional silica fiber. However, this transmission window has been expanded with the development of new fibers. In many new fibers, where the OH absorption around 1400 nm has been greatly reduced, there is a potential for optical amplifier configurations which can amplify between an entire optical operating range of from 1100 nm to 1700 nm.
One particular amplifier configuration that has been used to amplify wavelength band ranges greater than can be amplified with singular rare earth-doped amplifiers is the semiconductor optical amplifier. A semiconductor optical amplifier can provide gain over the entire operating transmission window of 1100 nm to 1700 xcexcm. For example, semiconductor optical amplifier components based on the material composite of
GaxIn1-xAsyPy-1 can provide gain within the range of 1000 nm to 1650 nm depending on the relative concentration of the constituent elements.
Optical amplification, including amplification affected by a semiconductor optical amplifier, relies on the known physical mechanisms of population inversion and stimulated emission. More specifically, amplification of an optical signal depends on the stimulated transmission of an optical medium from an inverted, excited state to a lower, less excited state. Prior to the actual amplification of the optical signal, a population inversion occurs, i.e., more upper excited states exist than lower states. This population inversion is affected by appropriately energizing the system. In semiconductor optical amplifiers, an excited state is a state in which there exists an electron in the conduction band and a concomitant hole in the valance band. A transition from such an excited state, to a lower state in which neither an electron nor a hole exists, results in the creation of a photon or a stimulated emission. The population inversion is depleted every time an optical signal passes through the amplifier and is amplified. The population inversion is then reestablished over some finite period of time. As a result, the gain of the amplifier will be reduced for some given period of time following the passage of any optical signal through the amplifier. This recovery of time, is typically denoted as the xe2x80x9cgain-recovery timexe2x80x9d of the amplifier.
In contrast to erbium-doped amplifiers, or other rare earth-doped amplifiers, semiconductor optical amplifiers are smaller, consume less power and can be formed in an array more easily. Accordingly, semiconductor optical amplifiers are important in applications such as loss compensation for optical switches used in multi-channel optical transmission systems or optical switchboard systems.
Two major drawbacks are associated with semiconductor optical amplifiers. The first drawback is that the noise figure associated with semiconductor optical amplifiers is significantly high. While all amplifiers degrade the signal-to-noise ratio of the amplified signal because of spontaneous emission that is added to the signal during amplification, the noise figure associated with semiconductor optical amplifiers is extremely problematic. The reason for this is two-fold. Firstly, low loss coupling from an optical fiber to a semiconductor optical amplifier is difficult due to the mode field mismatch between the devices. Specifically, coupling losses due to mode field mismatch are generally in the region of about 2 dB or more. The coupling loss generated from coupling an optical fiber with a semiconductor optical amplifier is directly added to the intrinsic (or internal) noise figure of the device. Secondly, increasing the gain in a semiconductor optical amplifier requires increasing the length of the device and/or the optical confinement factor. While increasing the length of the device and/or the optical confinement factor allows for high gain in the optical signal, this also results in high gain for the amplified spontaneous emission generated within the semiconductor optical amplifier. Above a certain device length, this amplified spontaneous emission will saturate the device, thereby limiting the achievable gain. Equally important, the backward traveling amplified spontaneous emission results in a reduction in the carrier density at the input of the device, thereby leading to a low inversion ratio within that device. As with any kind of optical amplifier, a low inversion ratio at the input of the amplifier results in an increased noise figure. In the past, the best achievable intrinsic noise figure for semiconductor optical amplifiers is around 4 dB for devices based on multiple quantum well structures, and around 5 dB for devices based on bulk guiding structures.
A second problem associated with semiconductor optical amplifiers is signal cross-talk resulting from cross-gain modulation. Signal cross-talk arises because the saturation output power of the semiconductor optical amplifier is lower than that of the fiber based amplifiers, and because the gain recovery time is on the same time scale as the data repetition rate. Thus, a semiconductor optical amplifier amplifying multiple signals with a combined input power greater than the input saturation power will superimpose cross-talk caused by gain modulation between the relative channels.
This invention meets the need for an optical amplifier that offers the benefit of a relatively large gain and optical signal strength together with a substantially low noise figure.
The invention relates to an optical signal amplifier that includes two semiconductor optical amplifiers separated by an optical signal isolator. More specifically, the present inventive optical amplifier makes use of the significant gain typically associated with semiconductor optical amplifiers while reducing the significant noise figure typically associated with semiconductor optical amplifiers.
In a preferred embodiment, an optical amplifier includes a first semiconductor optical amplifier having an input for receiving an optical signal, and an output, wherein the optical signal is amplified by the first semiconductor optical amplifier resulting in a first amplified optical-signal. The optical signal amplifier also includes an optical signal isolator having input in optical communication with the output of the first semiconductor optical amplifier, and an output, wherein the optical signal isolator receives the amplified signal from the first semiconductor optical amplifier and allows for transmission of an optical signal in a single direction. The optical signal amplifier further includes a second semiconductor optical amplifier having an input for receiving the amplified optical signal from the optical signal isolator, and an output, wherein the amplified optical signal is amplified by the second semiconductor optical amplifier resulting in a second twice amplified optical signal.
In addition, embodiments of the optical signal amplifier include an amplified spontaneous emissions filter located within the optical signal amplifier so as to prevent amplified spontaneous emissions lying outside the used gain band from propagating forward to the second semiconductor optical amplifier.
Further, embodiments of the optical signal amplifier include a gain flattening filter positioned within the optical signal amplifier and located between the first semiconductor optical amplifier and the second semiconductor optical amplifier to produce a flattened gain over a desired spectral range.
Other embodiments include an optical communications system that utilizes the optical signal amplifier, as well as a method for utilization of the inventive optical signal amplifier.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined in the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of the specification. The drawings illustrate various features and embodiments of the invention which, together with their description, serve to explain the principles and operation of the invention.